Guardrails are commonly provided about United States highways in areas where it is desirable to prevent vehicles from leaving the highway, e.g., at elevated portions of highway or between opposing lanes of traffic. Such guardrails can be generally classified into one of two performance categories, rigid and non-rigid. Rigid guardrails are not intended to deflect upon impact, and are instead intended to constrain the vehicle and redirect it onto the roadway. As an example, trapezoidal concrete slabs are commonly used to provide rigid guardrails between adjacent lanes of traffic. Non-rigid guardrails are intended to deflect upon impact so as to absorb and dissipate kinetic energy from an oncoming vehicle without overly damaging the vehicle and harming its passengers.
The most common non-rigid guardrail system is the w-beam guardrail, a hot-rolled steel rail having a w-shaped cross-section which is galvanized (zinc coated) for corrosion protection. W-beam guardrails are installed generally parallel to the highway on posts sunken into the ground, with the ends of adjacent rails being overlapped and bolted to the posts. A standard w-beam rail is sized, configured, and supported so as to allow up to approximately 1 meter of deflection when struck by a vehicle at highway speed. The w-beam primarily dissipates impact energy via several mechanisms: plastic flexural deformation of the rail; deformation and breakage of support posts; and the "plowing" of support posts through the ground. It is estimated that 600-800 million feet of w-beam guardrail is currently installed in the United States.
However, the w-beam guardrail suffers from several deficiencies. Its energy dissipation characteristics are such that substantial vehicle damage often occurs upon impact. Owing to recent trends toward increasing vehicle size, there is mounting concern that standard steel w-beams provide inadequate collision protection for today's traffic. The w-beam also has significant installation and replacement costs. Because of its weight and configuration, w-beam rails are difficult to transport, install, and remove by anything less than multi-person road crews, and they therefore tend to incur high labor costs. Costs can be further enhanced by the time and labor cost of closed traffic lanes and traffic rediversion during installation and replacement. It is notable that w-beam rails are generally not repaired after collisions because damage tends to be of such a permanent nature that repair is not cost-effective.
Despite these disadvantages, use of the w-beam guardrail is almost universal because appropriate alternatives are lacking, and it is otherwise viewed as providing an acceptable balance between its drawbacks and its benefits. As for its benefits, the w-beam guardrail has relatively low material cost in comparison to alternative guardrail systems, and it requires relatively low maintenance over its lifetime if it does not experience vehicle damage. If no collisions occur, the average operating lifetime of a w-beam rail is approximately 20 years, with lifetime mainly being determined by the corrosiveness of the beam's environment (e.g., whether the adjacent highway is salted in winter months). Nevertheless, since an estimated 3% of the existing w-beam guardrails require replacement each year, it is evident that installation and repair costs are significant.
As a result of the foregoing considerations, there is great interest in developing alternatives to the steel w-beam. Since plastics and composite materials have significantly different energy dissipation properties than metals, one area of interest to researchers is the possibility of developing plastic and composite guardrail systems. Examples of several such systems follow.
U.S. Pat. No. 3,317,189 to Rubenstein illustrates generally cylindrical and semicylindrical guardrails which are predominantly made of a rubber/concrete composite. These composite guardrails can also include a composite fiber-reinforced surface layer. Glass reinforcing cables may also extend through the length of the rail.
Several patents then propose plastic or composite guardrails which serve as hollow vessels for containing liquids or other energy-absorbing material. During impact, the vessels deflect and the filler material provides the majority of the energy dissipation. As examples, U.S. Pat. No. 4,681,302 to Thompson illustrates hollow plastic guardrail sections which may be joined end-to-end, and which may be filled with water to enhance energy dissipation. U.S. Pat. No. 3,540,699 to Guzzardella describes a guardrail having similar operation. U.S. Pat. No. 4,307,973 to Glaesener illustrates a guardrail having a sheet-metal shell filled with synthetic resin foam. U.S. Pat. No. 4,138,095 to Humphrey illustrates a guardrail having a hollow plastic base which may be filled with ballast and draped with baglike impact shields filled with sand or other granular material.
Other references then describe the testing and/or use of composite guardrails which are intended for mounting between supports along roadsides to function in the same manner as standard w-beam guardrails. The McDevitt and Dutta paper entitled "New Materials for Roadside Safety Hardware" (1992 Materials Engineering Congress of the American Society of Civil Engineers Materials Division, Atlanta, Ga., Aug. 10-12, 1992) notes the production of glass fiber-reinforced plastic w-beams. The March, 1995 issue of the journal Plastics World (at page 13) proposes the use of pultruded glass fiber-reinforced plastic tubes as highway guardrails. The 1996 ASME publication "Damage Evolution and Progressive Failure in Composite Material Highway Guardrails" by Gentry et al. describes a study of prototype guardrails formed of stock pultruded glass-fiber reinforced plastic bars and tubes which were bonded together to form beam-like guardrails.
Other proposed guardrail systems address the problem of inadequate energy dissipation in steel w-beams by utilizing plastic or composite support structures for the w-beams. U.S. Pat. No. 3,360,244 to Bucher illustrates the use of a series of plastic tubes mounted between a w-beam guardrail and its support posts. One or more tubes may be crushed when the w-beam is struck, and the crushed tube(s) may subsequently be replaced. U.S. Pat. No. 5,660,375 to Freeman describes a composite guardrail support post which is primarily designed to alleviate environmental concerns that arise where wooden support posts are used, but which is also stated to take safety concerns (i.e., impact behavior) into account.
However, none of the aforementioned plastic and/or composite guardrail systems are in widespread permanent use along U.S. highways. In general, they do not offer suitable energy dissipation characteristics at low enough cost that their substitution for steel w-beams is justifiable. Plastic and/or composite guardrails usually have higher material and production costs than steel w-beams, and they then require installation costs similar to those encountered with steel w-beams, making the composite guardrails overall significantly more expensive than w-beams. Installation is particularly expensive for the aforementioned vessel-type guardrails which require filling with energy-absorbing materials at the point of installation, since these are bulky and require time-consuming filling steps. The aforementioned composite beam-type guardrails also tend to incur high installation costs because they generally cannot be simply bolted to support posts in the same manner as steel w-beams. They have a greater tendency to fail at the bolts during impact, and therefore require specialized mounting structures and/or steps which significantly increase their costs.